1. Field of the Invention
The present invention relates to a lock mechanism of an electric connection box which is mounted in an automobile and the like.
2. Description of the Related Art
An electric connection box for housing fuses, relays and other wire harness components is mounted in a vehicle. In the case where a body of such an electric connection box houses therein a separate part such as a fuse box, a lock unit configured to fix the separate part to the box body is provided therebetween as described in Japanese Unexamined Patent Publication No. 2004-312844 and Japanese Unexamined Patent Publication No. 2005-86865.
As an example of such a lock unit, a lock mechanism 150 shown in FIGS. 1A, 1B, 2A, and 2B includes a flexible lock arm 151 provided on an inner wall 118 of a box body 110, and is configured to fix a part 120, which is a counterpart component, to the box body 110 with a lock projection 155 on the flexible lock arm 151 engaged with the part 120.
The lock mechanism 150 includes: the lock arm 151 which is provided on the inner wall 118 of the box body 110 so as to protrude obliquely upward from the inner wall 118 in a cantilever manner; and the lock projection 155 which is provided to the lock arm 151 on the surface thereof facing the part 120 housed in a part compartment so as to protrude from the surface. The lock mechanism 150 is configured to lock the part 120 when putting the part 120 into the part compartment, as follows. Specifically, the lock arm 151 is temporarily bent due to the interference between the lock arm 151 and a portion of the part 120. Then, after the interference portion on the part 120 passes the lock projection 155, the lock arm 151 returns to the original state, whereby the lock projection 155 comes to engage with and locks the part 120. In a top view of the lock arm 151, a wall portion 155b, which is provided on the lock projection 155 and configured to engage with the part 120, protrudes a distance d from the top portion of the lock arm 151. Therefore, when the part 120 is inserted, the protruding part comes in contact with the part 120.
The lock arm 151 can be divided in the longitudinal direction thereof into: a base end region 151a which is fixed to the inner wall 118; upper end regions 151c and 151d located in the free end which interferes with the part 120 and receives pressure applied for unlocking operation; and a middle region 151b which is located within a predetermined distance range between the base end region 151a and the upper end regions 151c and 151d. The lock arm 151 having such a configuration is usually formed to have approximately the same flexural rigidity throughout all the regions.
In the lock arm 151 shown in the drawings, ribs 156 are provided on the lower surface in the base part attached to the inner wall 118. All the regions of the lock arm 151 except for the region having the ribs 156 formed therein are formed to have the cross section of a squared U shape with ribs 152 provided on the both edges of the plate-shaped lock arm 151. By employing such a configuration, the flexural rigidity of the entire lock arm 151 is set approximately uniform.
Accordingly, as shown in FIG. 2B, when a unlocking operation force F is applied to the upper end region of the lock arm 151, the whole lock arm 151 is tilted as indicated by an arrow C with the base end side as a supporting point. With the lock arm 151 in this state, the part 120 can be released from the lock projection 155, and therefore the part 120 can be pulled upwards.
However, in the case where all the regions of the lock arm 151 in a longitudinal direction thereof have the same flexural rigidity as described above, the approximately entire lock arm 151 is to be bent according to a certain spring constant until the amount of elastic displacement reaches an elastic bending limit N4 as shown in a characteristic chart diagram in FIG. 3 illustrating the relationship between a force (a force required for locking and unlocking operations) and the amount of elastic displacement (the amount of bending of the lock arm 151). Accordingly, such a configuration raises a problem that a larger force will have to be applied for locking and unlocking operations if the flexural rigidity of the lock arm 151 is increased in order to enhance the lock retention. The configuration also raises a problem, on the other hand, that the lock retention will be weakened if the flexural rigidity of the lock arm 151 is reduced so as to eliminate the need for applying a large force for locking and unlocking operations. Hence, there has been a trade-off between the operability for locking and unlocking and the lock retention in a locked state.